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A previously developed mathematical model with parameters fitted to experimental data was used to study effects of material properties and experimental conditions on single gas permeance ratios of MFI membranes. The model showed that single gas permeance ratios are highly dependent on substrate morphology, feed pressure, crystallographic orientation and defects in the film. It was found that the pore size and the thickness of the substrate affected permeance ratios, due to mass transfer resistance in the substrate. The applied feed pressure also had a significant effect on the permeance ratios. This is due to differences in mass transfer resistance of the substrate and adsorption characteristics with varying feed pressures. The crystallographic orientation of the zeolite film also affected permeance ratios due to changes in diffusivity with varying orientation of the crystals in the film. Finally, the effect of defects was investigated. As expected, it was found that the permeance ratios decreased when more defects were added in the model. However, if the membrane is not very defective, the permeance ratio is much more affected by the substrate and by variation in pressure drop than by defects. The results in the present work show that single gas permeance ratios cannot be used directly as a benchmark of membrane quality unless all other parameters are kept constant. |
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